The present invention relates generally to an apparatus and method of monitoring the air pressures of tires on a vehicle. More particularly, this invention pertains to an air pressure monitoring apparatus having transmitters installed in the individual tires to wirelessly transmit data about the air pressures of the tires and a receiver mounted on the body frame of a vehicle to receive data from the transmitters. The invention also relates to a method of monitoring communication states between the transmitters and the receivers.
Wireless tire air pressure monitoring apparatuses for allowing a driver to check the air pressures of vehicle tires have been proposed. Such a monitoring apparatus includes transmitters installed on the wheels of the respective tires and a receiver mounted on the body frame of the vehicle. Each transmitter measures the air pressure of the associated tire and wirelessly transmits data including the measured pressure value. The receiver receives data transmitted from the transmitters and displays information about the air pressure of each tire on a display located in front of the driver""s seat.
In general, each transmitter measures the air pressure of the associated tire at predetermined time intervals (e.g., fifteen seconds). The transmitter also regularly transmits data every time the air pressure has been measured a predetermined number of times (e.g., forty times). If the interval for measuring the air pressure is fifteen seconds, therefore, the transmitter transmits data at intervals of ten minutes. When the air pressure of the tire changes abruptly, however, the transmitter performs data transmission a predetermined number of times (e.g., four times) in succession in accordance with the interval for measuring the air pressure. That is, the transmitter operates in accordance with a regular transmission mode according to a predetermined transmission interval and an abnormal transmission mode, which is executed when the air pressure of the associated tire is abnormal.
Wireless communication between the transmitters and the receiver is less reliable than wired communication. This is particularly so for an air pressure monitoring apparatus, which is mounted on a vehicle with a metal body. The metal body is likely to affect radio waves. What is more, because each transmitter is installed on an associated tire, the positional relationship of the transmitter with respect to the receiver changes quickly as the vehicle moves. As the transmission antenna of the transmitter and the reception antenna of the receiver both have directivities, the reception precision significantly changes when the vehicle moves.
One way of improving the reliability of communication is to increase the output level of the radio waves. Because the output level of the radio waves is restricted by communication laws, however, the output level of radio waves is limited.
It is therefore important for an air pressure monitoring apparatus to always monitor the communication state between each transmitter and the receiver.
A conventional air pressure monitoring apparatus discriminates if communication is carried out normally by causing the receiver to monitor the data reception interval. When the transmitter is operating in regular transmission mode, for example, the receiver determines if communication is normal or abnormal based on whether or not data has been received at regular time intervals.
When the transmitter is operating in abnormal transmission mode, however, the receiver cannot determine whether data transmission according to the abnormal transmission mode has been carried out properly or if data sent in the abnormal transmission mode has surely been received. That is, the receiver can easily determine if data has been received at regular time intervals but cannot detect, otherwise, if data transmission according to the abnormal transmission mode has been performed unless it actually receives data. When there is an abnormality in the pressure, when data transmission has not been executed due to a failure in the transmitter or when transmission data from the transmitter has not been received due to a poor communication environment, the receiver cannot detect the communication failure or the abnormality.
The deterioration of the communication environment that prevents data reception is often temporary. When the communication environment improves, therefore, the receiver can resume reception of data from each transmitter. When the received data has been transmitted in the abnormal transmission mode, however, the receiver cannot determine whether or not there has been any unreceived data before reception of that data. This delays the detection of a communication failure, which reduces the reliability of the air pressure monitoring apparatus.
Accordingly, it is an objective of the present invention to provide a tire air pressure monitoring apparatus and monitoring method which can accurately and reliably determine the communication state between each transmitter and a receiver.
To achieve the above objective, the present invention provides a tire air pressure monitoring apparatus having a transmitter for wirelessly transmitting data concerning the internal pressure of a tire on a vehicle and a receiver for receiving the data. The apparatus comprises generation means provided in the transmitter and determination means provided in the receiver. The generation means assigns a value to a variable according to a predetermined sequence every time the transmitter sends data. The transmitted data includes a current value of the variable. The determination means determines whether there has been a communication failure based on the value of the variable in the received data.
Also, the present invention provides a method of monitoring a communication state between a transmitter and a receiver. The method comprises: assigning a value to a variable in accordance with a predetermined sequence every time the transmitter sends data, wherein the transmitted data represents at least a physical status and the current value of the variable; receiving data with the receiver; and determining whether there has been a communication failure based on the value of the variable in the received data.
The present invention further provides a method of monitoring communication states among a plurality of transmitters and a receiver. The method comprises: assigning, for each transmitter, a value to a variable in accordance with a predetermined sequence every time each transmitter sends data, wherein the transmitted data represents at least a physical status and the current value of the variable; receiving data with the receiver; finding a number of skipped receptions between the previous data reception and the current data reception for each transmitter based on the variable values in the associated data; and determining whether each transmitter is malfunctioning by analyzing the numbers of skipped receptions for all of the transmitters.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.